Due to the process maturity and low fabrication cost, the anodic bonding of borosilicate glass with silicon (Si) is widely used in constructing current microelectricalmechanical systems (MEMS) based sensors and actuators. Various types of pressure sensors which use this technology include differential pressure sensors, front side absolute pressure sensors, and backside absolute pressure sensors.
The surface layers of borosilicate glass, however, react with moisture at an elevated temperature, such as in the environment of 85% relative humidity and 85° C. temperature, which is one of the typical test conditions in the automotive industry. It is this reaction that results in device instability, causing output shift. In some cases, this output shift is not recoverable, and becomes a permanent output change, which presents a severe reliability issue.
The primary cause in this instability is due to the reaction of the glass surface with moisture, forming highly stressed surface layers that may result in a glass surface-plastic deformation. There are two major reaction areas in typical MEMS devices that may cause a significant output instability issue, they are the die attach surface and the Si-glass interface. The bottom surface of the glass pedstal for all three types of pressure sensors (differential, front side absolute, and backside absolute) is normally attached to a housing substrate by using a die attachment material. The reaction of glass with the moisture in the die attachment material may degrade the bonding force with, or may even cause delamination from, the die attachment material after long exposure in a hot and humid environment. The Si-glass bonding interface is normally exposed to the moisture around the edges of the device, where the exposed glass may also react with moisture, causing a change in the bonding force with the Si. All of these changes may cause device output shift with high thermal hysteresis, and become a permanent change in many cases.
Accordingly, there exists a need for an MEMS device which reduces or prevents device output shift when exposed to high humidity under elevated temperatures.